1. Technical Field of the Invention
The present invention is generally directed to photonic switching devices, and it particularly relates to ultra-fast all-optical switching devices using light bullets as optical pulses and made of nonlinear optical materials, including highly nonlinear optical glasses, semiconductor crystals and/or multiple quantum well semiconductor materials. More specifically, the present invention relates to an all-optical switching process in a planar slab waveguide.
2. Description of the Prior Art
When a pulse of light travels in a linear dispersive medium its length increases due to group-velocity dispersion. Depending on the intensity of the pulse and the material properties of the medium, nonlinear effects on the pulse shape, called self-phase modulation, can also be significant. Nonlinear effects are characterized by a nonlinear relationship between the polarization density and the electric field; an example is the Kerr effect. The interplay between self-phase modulation and group-velocity dispersion can therefore result in an overall pulse spreading or pulse compression, depending on the magnitudes and signs of these two effects.
Under certain conditions, an optical pulse of prescribed shape and intensity can travel in a nonlinear dispersive medium without altering its shape, as if it were traveling in an ideal linear non dispersive medium. This occurs when the group-velocity dispersion fully compensates for the effect of self-phase modulation. Such pulse-like stationary waves are called solitary waves. Optical solitons are special solitary waves that are orthogonal, in the sense that when two of these waves cross one another in the medium, their intensity profiles are not altered, and only phase shifts are imparted as a result of the interaction, so that each wave continues to travel as an independent entity.
At a certain level of intensity and for certain pulse profiles, the effects of self-phase modulation and group-velocity dispersion are balanced so that a stable pulse, the soliton, travels without spread. The mathematical analysis of this phenomenon has so far been based on approximate solutions of Maxwell's nonlinear wave equations.
As used herein, the term soliton refers to generic solutions describing pulses that propagate without substantial change, and may be temporal or spatial. Spatial solitons are monochromatic, self-guided beams that are localized spatially in the transverse plane. They travel in a nonlinear medium without altering their spatial distribution, as a result of the balance between diffraction and self-phase modulation. Spatial solitons are the transverse analogs of temporal or longitudinal solitons.
Optical pulses including solitons, may be used for photonic switching and computing. Switching is an essential operation in communication networks. It is also a basic operation in digital computers and signal processing systems. The current rapid development of high-data-rate fiber-optic communications systems has created a need for high capacity repeaters and terminal systems for processing optical signals, and therefore, a need for high-speed photonic switches. Similarly, the potential for optical computing can optimally be realized if large arrays of fast photonic gates, switches, and memory elements are developed.
As used herein, a switch is a device that establishes and releases connections among transmission paths, such as in communication or signal-processing systems. A control unit processes the commands for connections and sends a control signal to operate the switch in the desired manner.
A switch is generally characterized by the following parameters:
Size: number of input and output lines. PA0 Directions: whether data can be transferred in one or more directions. PA0 Switching time: time necessary for the switch to be reconfigured from one state to another. PA0 Propagation delay time: time taken by the signal to cross the switch. PA0 Throughput: maximum data rate that can how through the switch when it is connected. PA0 Switching energy: energy needed to activate and deactivate the switch. PA0 Power dissipation: energy dissipated per second in the process of switching. PA0 Insertion loss: drop in the signal power introduced by the connection. PA0 Crosstalk: undesired power leakage to other lines.
Optical signals may be switched by the use of electrical, acoustic, and magnetic switches. For instance, in electro-optic switches, the optical signals are converted into electrical signals using photodetectors, switched electronically, and then converted back into light using LEDs or lasers. These optical/electrical conversions introduce unnecessary time delays and power loss, in addition to the loss of the optical phase caused by the process of detection. Therefore, direct optical switching is clearly preferable to non optical switching.
In an all-optical (or opto-optic) switch, light controls light with the help of a nonlinear optical material. Nonlinear optical effects may be direct or indirect, and may be used to make all-optical switching devices. All-optical switching devices have the capability of switching at much higher rates than non optical switching devices. Exemplary all-optical switching devices are described in Friberg, S. R., Weiner, A. M., Silberberg, Y., Sfiz, B. G., and Smith, P. W., "Femtosecond switching in a dual-core-fiber nonlinear coupler", Optics Letters, Vol. 13, No. 10, pp. 904-906, October, 1988.
Currently, there exists a number of all-optical switching devices, including the birefringent-fiber polarization switch, the optical-fiber Kerr gate, the two-core-fiber nonlinear directional coupler, the birefringent single-core-fiber, the nonlinear fiber-loop mirror, the soliton dragging logic gate, the bistable nonlinear optical switching device, the spatial soliton beam switch in a planar waveguide, the nonlinear polarization switch in a semiconductor waveguide including a multiquantum well waveguide, the semiconductor interferometer switch, the nonlinear Bragg semiconductor waveguide switch, and the bistable optical switch. A general description of such all-optical devices can be found in Saleh, B. E. A., and Teich, M. C., "Fundamentals of Photonics", John Wiley, 1991; and Agrawal, G. P., "Nonlinear Fiber Optics", Academic Press, 2nd Ed., 1995.
Spatial and temporal solitons have been produced in the laboratory and used for all-optical switching. Bell, T. E., in an article entitled "Light that acts like `natural bits`, IEEE Spectrum, pp. 56-57, August 1990, introduces the different types of solitons (temporal and spatial), and the possible uses of these solitons mainly in fiber optic communications, i.e., temporal solitons for long range communications and spatial solitons for optical switching. Reference is also made to Aitchison, J. S., Weiner, A. M., Silberberg, Y., Oliver, M. K., Jackel, J. L., Leaird, D. E., Vogel, E. M., and Smith, P. W. E., "Observation of spatial optical solitons in a nonlinear planar waveguide," Optics Letters, Vol. 15, No. 9, pp. 471-473, May 1, 1990, which describes the power levels for creating spatial solitons beams in the laboratory, and the process of making a corresponding waveguide.; and to Aitchison, J. S., Silberberg, Y., Weiner, A. M., Leaird, D. E., Oliver, M. K., Jackel, J. L., Vogel, E. M., and Smith, P. W. E., "Spatial optical solitons in planar glass waveguides," Journal Optical Society of America B, Vol. 8, No. 6, pp. 1290-1297, June 1991, which further describes the interaction between solitons beams, and the type of glass material used (Schott B270 glass). Both of these articles to Aitchison et al. are incorporated herein by reference. This type of glass has a relatively weak nonlinear index of refraction (n.sub.2) with a nonlinearity value n.sub.2 =3.4.times.10.sup.-16 cm.sup.2 /W.
The power requirements for an optical soliton decreases as the strength of the nonlinear index of refraction increases. Therefore, the use of highly nonlinear glasses is preferable because they have larger nonlinear indices of refraction, and will significantly reduce the power requirements for the solitons.
Borrelli, N. F., Aitken, B. G., and Newhouse, M. A., in an article entitled "Resonant and non-resonant effects in photonic glasses," Journal of Non-Crystalline Solids, pp. 109-122, Vol. 185, (1995), which is incorporated herein by reference, publishes the result of research done on glasses and polymers that exhibit large nonlinear susceptibilities, and list these materials and their characteristic properties on page 111. However, this article does not address the future possibility of generating light bullets using the listed materials. Reference is made to pages 120 and 121, "4. Conclusion/future".
In a nonlinear optical material, temporal soliton pulses are confined in the direction transverse to propagation by propagating in a fiber. A more maneuverable temporal soliton would be able to move in a transverse direction, such as in a planar slab waveguide. Such special types of solitons are referred to as "light bullets". Light bullets are essentially pulses of light which, when propagating in a nonlinear material, maintain their shapes under the effect of diffraction (spreading transverse to the direction of propagation), dispersion (spreading in the direction of propagation), and nonlinearity.
However, light bullets have so far only been studied theoretically, and have not yet been produced in a laboratory. Additionally, until recently, light bullets were believed to be unstable, unless the material is saturable. Reference is made to Silberberg, Y., "Collapse of optical pulses", Optics Letters, Vol. 22, pp. 1282-1284, Nov. 15, 1990, which states "the saturation of the nonlinear index (practically attainable only in gaseous systems)". The analysis in this article is based on the nonlinear Schrodinger equations, which, in turn, is an approximation of Maxwell's equations. In essence, the author stated that light pulses will collapse. However, this analysis resorted to an approximation which neglects higher order terms in resolving Maxwell's equations, and did not take into account factors that limit the collapse, such as higher order dispersion.
A computer simulation that uses the exact Maxwell's equations without any approximation, and which therefore automatically accounts for higher order terms of all orders that would occur in an approximate approach, was done and was published by Goorjian, P. M. and Silberberg, Y. "Numerical simulations of light bullets, using the full vector, time dependent Maxwell equations," Nonlinear Optics Topical Meeting, IEEE/Lasers and Electro-Optics Society and Optical Society of America, Waikoloa, Hawaii, Jul. 24-29, 1994.; and Goorjian, P. M. and Silberberg, Y., "Numerical simulation of light bullets, using the full vector, time dependent Maxwell equations," Integrated Photonics Topical Meeting, cosponsored by the Optical Society of America and IEEE/Lasers and Electro-Optics Soc., Dana Point, Calif., Feb. 23-25, 1995. Both of these articles are incorporated herein by reference.
This later study showed that light bullets are in fact stable, and that there is no need for saturating the material to obtain stability. This study also very briefly mentions that light bullets can deflect each others' travel paths upon collision. These light bullets will be on the order of 25 to 250 femtoseconds in duration, where one femtosecond is one millionth of one billionth of a second (10.sup.-15 second). This study proposed a mathematical model for light bullets and used a hypothetical nonlinear optical material with conjectured dispersion parameters.
None of the existing or previously proposed all-optical switching devices uses or proposes the use of light bullets in planar slab waveguides made from commercially available nonlinear optical glass. Several of the prior devices are relatively large physically or use relatively large optical pulses, as compared to the proposed device. In some of those prior devices, such as the two-core-fiber nonlinear directional coupler, the light pulses interact relatively weakly through evanescent waves. The spatial soliton devices suffer from the effects of dispersion on the pulses and the temporal soliton devices are confined to fibers and hence do not have the maneuverability of pulses in waveguides. None of the prior devices use light bullets, which are extremely small, maneuverable and do not degrade on propagation, (i.e., are self-sustainable).
Therefore, there is still a great and unsatisfied need for a practical realization of an ultra-fast all-optical photonic switching device utilizing light bullets. The material used to build this device should be readily available and relatively inexpensive to manufacture, and it should further exhibit characteristic parameters that are adequate for the production of light bullets.